The present invention is connected with transponder systems for wireless payment, e.g. for payment of a toll for vehicles. Q-Free ASA has manufactured such systems for many years. These systems are used in several countries named as the “Q-free box”. The expression “box” is related to the transponder element in this system provided in every individual vehicle. The transponder receives data from a device placed near the road, and as an answer it sends individual data back to the road device.
The technological development within this area has in the meantime moved towards active transponders operating with microwave radiation in the area of 5.8 Hz, that is wavelengths in the area of centimeters, which have a battery and an active communication controller. The transponder receives data as amplitude modulated radiation and sends data as phase modulated radiation.
The simplest embodiment of such a transponder is a diode coupled into an antenna, leading to amplitude demodulation by rectifying the carrier wave. By sending, a current is sent alternately in the diode, and its reflection coefficient is thus changing, and accordingly this gives phase modulation. The principle makes it possible to send without use of a local-oscillator on the transponder and it is known as “back-scattering”.
Because of the large production rate for such transponders it is a difficult task to make transponder antennas which have little scattering and which may be produced as simple and cheap as possible.
Known antennas which are easy to manufacture are microstrip antennas. These are antennas which are easily realizable on a substratum together with the rest of the circuitry. The problem regarding microstrip antennas is that they are based on resonance where a large e-field concentration along the edge of the antenna element arises towards the earth plane. The effectiveness of the antenna and the resonance frequency are very dependant on the dielectric constant in the substratum and the thickness of the substratum. Accordingly, a usual printed circuit board laminate, such as “FR-4” glass fibre laminate, is not suitable for the production of such antennas. Good microwave laminate based on PTFE (teflon) is the most common in use, but this laminate is expensive, complicated to manufacture and uses few environmentally friendly processes during the manufacturing.
Lately, laminates have become available that are something between glass fibre laminate (FR-4) and PTFE laminate, such as “ROGERS 4300”, but still this is not an alternative able to compete with standard laminate.